externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c598a9a517d7bdec8ba1a0bace3ef10' as 'externals/catch'

externals/catch/tools/misc/appveyorMergeCoverageScript.py

@@ -0,0 +1,9 @@
+#!/usr/bin/env python3
+
+import glob
+import subprocess
+
+if __name__ == '__main__':
+    cov_files = list(glob.glob('tests/cov-report*.bin'))
+    base_cmd = ['OpenCppCoverage', '--quiet', '--export_type=cobertura:cobertura.xml'] + ['--input_coverage={}'.format(f) for f in cov_files]
+    subprocess.check_call(base_cmd)